1. Field of the Invention
This invention relates to a displacement detection device, and in particular to a detection device for detecting displacement of a cantilever, which is employed in a Scanning Force Microscope (SFM).
2. Description of the Related Art
An Atomic Force Microscope (AFM) and a Magnetic Force Microscope (MFM) derived from a Scanning Tunnel Microscope (STM) are known as the Scanning Force Microscope.
The STM is well known as a device for observing the fine surface structure of a sample by using tunnel current. In the device, when a probe having its tip sufficiently pointed is moved toward the sample, electron clouds formed of an atom at the tip of the probe and of a atom at the surface of the sample become superposed on each other. At this time, if the probe and sample are electrically conductive, and a bias voltage is applied therebetween, tunnel current flows locally as a result of electron tunneling. The intensity of the tunnel current varies in accordance with the distance between the probe and sample. If the distance is approx. 1 nm, the intensity will vary by ten times whenever the distance varies by 0.1 nm.
On the other hand, in the AFM or MFM as a scanning force microscope, the detection device employed, which uses an elastically deformable member such as a cantilever, detects fine interactive forces acting upon atoms at the tip of a probe and at the surface of a sample and depending upon the distance between the atoms, i.e. a van der Waals force, a coulombic force, a frictional force, an absorbing force, a magnetic force, etc. In other words, the STM, AFM, and MFM detect an interactive force created between an atom at the tip of the probe and an atom at that part of the surface of the sample which is located in the vicinity of the probe tip. Thus, the structure of the surface of the sample is observed by means of a longitudinal resolving power determined from the distance between adjacent atoms, and a transverse resolving power determined from the degree of expansion of that atom at the probe tip which is involved in the detected interaction.
In the SFM, sample surface data of three dimensions or more can be obtained by moving the probe and sample relative to each other, thereby performing raster scan, and sampling a correlative force acting at each measuring point based on the transverse resolving power.
In the STM, in general, the servo system employed is controlled during scanning so as to keep the tunnel current or distance between a probe and a sample at a constant value, and servo outputs and measuring point data are displayed in a synchronous manner, thereby obtaining a three-dimensional image indicative of the unevenness of the sample surface.
Also in the AFM or MFM, the servo system is controlled during scanning so as to keep the force, acting upon the cantilever, at a constant value, and servo outputs and measuring point data are displayed in a synchronous manner, thereby obtaining a three-dimensional image indicative of the sample surface. In this case also, the three dimensional image indicates the configuration of the sample surface, if the sample contains a single kind of atoms, since the image is based on the distance between the probe and sample.
In the above SFMs, however, it is necessary to vary the material of the probe and/or the curvature of the tip of the probe in accordance with a sample to be measured. Specifically, the magnitude and direction of the force acting between an atom T of the probe and an atom S of the sample, vary in accordance with the distance therebetween and with the kinds of the atoms.
As regards the atom S of the sample, the magnitude and direction of the force vary in the following cases: a case where the sample consists of one kind of atoms, i.e., it is made of, for example, graphite or silicon crystal; a case where the sample consists of a plurality of kinds of atoms, i.e., it is made of, for example, a III-V compound having a crystalline structure of gallium (Ga) and phosphorus (P); a case where the sample is made by doping an impurity into silicon crystal or GaP crystal; a case where a defective has grown in crystal structure; a case where phase shift has occurred in structure; and a case where the state of the sample surface has varied in accordance with a change in temperature.
On the other hand, as regards the probe, if the distance between the probe tip and sample is a few tens nm, a plurality of atoms T of the tip will act upon a plurality of atoms S of the sample, even when the curvature of the tip is small. In a case where an adsorption material (e.g. oxygen) is attached to the tip of the probe, the combination of atoms is varied, resulting in a change in interactive force.
It is known that servo control to keep the interactive force at a constant value is effective in the case of using a cantilever as detection means for detecting various sample structures. However, if a given point of the sample is measured under the servo control, it is necessary to judge which force is exerted in accordance with the position of the probe, and to select a cantilever having an elastic force suitable for measuring the force under the condition. In general, a cantilever can have a desired elastic characteristic by changing its shape without changing its material. To detect the total force of various elements as described above means to detect, for example, that displacement of a cantilever which reflects a structure more complex and finer than a simple crystal structure, such as graphite or silicon crystal, which has been detected by the conventional STM or AFM.
Further, a cantilever for use in the AFM or MFM is preferably formed thin, long, and narrow, by using a material being as light as possible and having a large elastic coefficient, so that it could be greatly displaced by a fine force (interatomic force or magnetic force). However, a long cantilever inevitably has a low natural frequency, resulting in deteriorated responsiveness in scanning the unevenness of the sample. Moreover, it is difficult to eliminate noise due to external vibration. To avoid these, G. Bennig of IBM and C. F. Quate of Stanford University recommended forming of a cantilever of Si by lithography, deposition, etching, as well as an Integrated Circuit process. At present, a cantilever having a length of 100-200 .mu.m, a width of 10-40 .mu.m and a thickness of 0.3-0.6 .mu.m is available, and its eigenfrequency is 10-70 kHz and its force sensitivity is 0.004-0.04 N/m.
There is a method for detecting a displacement in the above-described cantilever. In the method, an STM is formed opposed to the reverse side of the cantilever (i.e., the side having no probe), and a displacement in the cantilever is detected based on a variation in tunnel current. Though the STM has a sufficient sensitivity for the distance between the probe and cantilever, accurate measurement still cannot be performed since an interatomic force exists even therebetween.
In another method, a light reflecting surface is provided on the reverse side of a tip portion of a cantilever, and a laser beam is reflected on the surface, thereby detecting, by a PSD (position sensitive device) a reflection angle varying in accordance with a displacement in the cantilever. In this method, however, if the incident angle of a beam is increased to enhance the sensitivity of the device, or if the distance between the cantilever and detector is increased in an optical lever method to enhance the same, the device must be made larger, so that its eigenfrequency will inevitably be reduced, and the detection accuracy will contrary be reduced.
In a furthermore method of this kind, light emitted from a laser is divided into two light beams, i.e., a reference beam and a detection beam, and the detection beam is irradiated onto an optical reflecting mirror provided on the reverse side of a tip portion of a cantilever, thereby making the beams interfere with each other and detecting the resultant coherent light. To obtain a good sensitivity, the light passage of a reference light system must have the same length as that of a detection light system so as to eliminate the influence of the ambient factors such as temperature and pressure, which makes the device complex. Further, if the reference light system is provided apart from the detection light system, it will be very difficult to equalize the eigenfrequency of their respective light passages to each other, resulting in a reduction in sensitivity due to the influence of the ambient factors.
The above-described detection devices must be made large and heavy, and hence have the disadvantage that detection signals may be buried in external noises even if the device has a high detection accuracy by virtue of employment of a fine cantilever.